This invention relates to a radio frequency resonator coil suitable for magnetic resonance imaging. The radio frequency resonator coil is a well known device that is useful for imaging regions of a human patient, such as the head and neck. Other useflI applications for this invention include imaging other portions of the human anatomy or for imaging non-human subjects.
Magnetic resonance imaging (MRI) is an imaging technique that may be used to produce high resolution images of the interior of the human body, for example, for the purpose of medical diagnosis. Interior images of the human body are produced based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum. The emission of energy is then correlated to the attenuation spectrum of the various tissues in the human body.
Typically, magnetic resonance imaging is performed by placing a patient in a constant magnetic field, B0. A radio frequency excitation pulse may then be transmitted to the examined region. The excitation pulses cause magnetic moment alignment of atomic nuclei. Upon removal of the excitation pulses, the nuclear moments begin to realign with the constant magnetic field, B0. During this realignment period, the nuclear moments emit radio frequency signals characteristic of the magnetic field and of the particular chemical environment in which the nuclei exist.
An RF coil may be used to both transmit the excitation pulses and receive the signals from the nuclei. Alternatively, one RF coil may be used to transmit the excitation pulses and another separate coil to receive the signals from the nuclei. RF coils of various types are known.
For purposes of the following, coordinate references (X, Y and Z) are sometimes discussed. As used herein, the Z-axis is oriented along the main magnetic field, B0. When reference is made to homogeneity, it is in reference to the homogeneity of the field pattern along the Z-axis, at any point in the directions of the Y and X axes.
One type of RF coil, the birdcage resonator coil, is known for its homogeneity in the XY image plane, as well as its high signal-to-noise ratio performance. Such devices, however, may still exhibit shortcomings. For example, in the XZ and YZ image planes, the field pattern is not as homogeneous as it may be in the XY image plane. Homogeneity in the XZ and YZ image planes is desirable for high resolution imaging. Increased homogeneity in the YZ or XZ image planes of the conventional birdcage coils may be achieved by extending the length of the coil. This increases the volume of the coil, however, and therefore reduces the signal-to-noise performance of the coil.
U.S. Pat. No 5,602,479 to Srinivasan et al. (xe2x80x9cthe Srinivasan patentxe2x80x9d), the contents of which are incorporated herein by reference, describes a birdcage coil where the z-axis conductors converge to a single point at the superior end of the coil. Birdcage coils having this characteristicxe2x80x94the convergence of the z-axis conductors to a single pointxe2x80x94may be referred to herein as xe2x80x9cdome resonators.xe2x80x9d Srinivasan further shows combinations of conductors using overlapped resonators, where the conductor junction points of all the z-axis conductors of the dome resonator are merged to a common point at one end which forms a virtual ground point.
A disadvantage of the devices shown in the Srinivasan patent is that the convergence of the junction points to form a virtual ground forces the resonator magnetic flux density to be highest at the convergent dome end, and fall off non-homogeneously as a function of distance from the dome end. The Srinivasan patent also discusses methods of distributing currents in a dome resonator between first and second rings or changing the convergence angle to influence homogeneity. The disadvantages of these methods, however, are that they require careful component selection to distribute the currents appropriately, they result in a coil that is too long for optimum signal-to-noise ratio, and they require a virtual ground convergence point.
An article entitled xe2x80x9cA 3xc3x973 Mesh Two Dimensional Ladder Network Resonator for MRI of the Human Head,xe2x80x9d Meyer et. al., Journal of Magnetic Resonance, Series B 107, 19-24 (1995), the contents of which are incorporated herein by reference, demonstrates that the resonant modes of the planar coil pairs are degenerate modes, and provides a saddle pair coil configuration at the non-domed end of the resonator, and a coplanar loop configuration at the domed end. Disadvantages of such an arrangement may include non-optimized homogeneity, and reduced signal-to-noise performance compared to devices made in accordance with the present invention.
An article entitled xe2x80x9cA Novel Multi-segment Surface Coil for Neuro-Functional Magnetic Resonance Imaging,xe2x80x9d Lin et. al., Magnetic Resonance in Medicine, 39: 164-168 (1998), the contents of which are incorporated herein by reference, demonstrates the resonant modes of the planar coil pairs are degenerate modes, as two orthogonal half-loop modes and causes the magnetic flux density to be most concentrated at the closed superior end or apex end of the resonator, thereby producing strong inhomogeneity. The device described by Lin et al. also requires closed end convergence at the most superior end of the conductors for the resonance modes to operate.
It is desirable to have improved homogeneity and, simultaneously, high signal-to-noise ratio performance, particularly in the XZ and YZ image planes, over that provided by known coils and methods. It is also desirable to improve homogeneity without impairing the signal-to-noise performance of a birdcage coil. It would therefore be desirable to have an improved birdcage coil.
This invention describes a novel apparatus and method for improving the XZ and YZ inhomogeneity of birdcage coils, without effecting the length of the coils.
The most homogeneous section of a conventional birdcage coil is within the center third of the length of the structure. Therefore to achieve maximum homogeneity within this region, a length to diameter ratio of two to one is typically used. The point of diminishing return for homogeneity has empirically been shown to be when the length-to-diameter ratio reaches two-to-one. To achieve maximum signal-to-noise ratio performance for human head imaging however, the ratio of the length-to-diameter is approximately one to one. Therefore, prior to the present invention, conventional birdcage coils could not simultaneously provide maximum signal-to-noise performance for human head imaging while achieving maximum homogeneity, especially over the region of the brain, because of the conflicting physical restraints placed on the coil by these two requirements.
A preferred embodiment of the present invention meets the need for an MRI coil that provides the high signal-to-noise ratio performance found in a normal length resonator coil combined with the homogeneity of a long resonator coil. In particular, improved homogeneity is achieved without increasing the length of the coil, thereby maintaining a high signal-to-noise ratio. Moreover, the preferred embodiments of the present invention meet the requirement of improved XZ and YZ image plane homogeneity for shorter, higher signal-to-noise ratio, birdcage coils by tapering the coil to optimize the magnetic field homogeneity. This technique may provide an improved signal-to-noise ratio over even a short birdcage coil of conventional design, especially in the region of the brain.
In accordance with a first aspect of the present invention, a method is provided for increasing the homogeneity in the magnetic field without increasing the length to diameter ratio of the coil. The method provides a coil with superior signal-to-noise performance and increased homogeneity in the XZ and YZ image planes.
In accordance with a second aspect of the present invention, a tapered birdcage coil is provided. The coil structure includes critically tapering the most superior end of the coil, and varying the radius of the taper and the end ring diameter, until the maximum homogeneity is realized in the field pattern of the coil.
In accordance with a third aspect of the present invention, a tapered birdcage coil is provided. The coil structure includes critically tapering both the superior end of the coil and the inferior end of the coil, and varying the radius of the taper and the end ring diameters, until the maximum homogeneity is realized in the field pattern of the coil. The tapered birdcage coil provides improved homogeneity and good signal-to-noise ratio performance.
It is an object of the present invention to provide the homogeneity of a longer birdcage coil, with the signal-to-noise characteristic of a shorter birdcage coil.